Compositions containing dithionites

ABSTRACT

1. A SUBSTANTIALLY ANHYDROUS COMPOSITION COMPRISING A MAJOR PROPORTON OF A METALLIC DITHIONITE AND AN EFFECTIVE AMOUNT OF A STABILIZER THEREFORE WHICH IS A NOMSURFACTANT COMPOUND CONTAINING AT LEAST 5 CARBON ATOMS AND AT LEAST ONE GROUP OF THE STRUCTURE:   -CO-N&lt;,   WHEREIN THE UNSATISFIED VALENCIES ARE FILLED BY OXYGEN, HYDROGEN OR CARBON ATOMS, SAID COMPOUND BEING MONOMERIC OR CONTAINING NOT MORE THAN 10 REPEATING UNITS AND HAVING A MOLECULAR WEIGHT OF UP TO 600.

United States Patent 3,840,644 COMPOSITIONS CONTAINING DITHIONITESRoland Albert Leigh, 9 Beckwith Road, Harrogate, Yorkshire, England NoDrawing. Filed Jan. 10, 1972, Ser. No. 216,799 Claims priority,application Great Britain, Jan. 14, 1971, 1,930/71; July 1, 1971,30,937/71; July 14, 1971 Int. Cl. C01b 17/66 US. Cl. 423265 28 ClaimsABSTRACT OF THE DISCLOSURE Metallic dithionite compositions stabilizedby incorporation of a non-surfactant, non-polymeric compound containingat least carbon atoms and comprising at least one group of the structurewhere the unsatisfied valencies are filled by oxygen or carbon atoms.

This invention relates to compositions comprising a metallic dithionite,particularly sodium dithionite.

Aqueous solutions of dithionite are used extensively in the textile andpulp and paper industries as bleaching agents, as reducing agents forvat dyes and as means of destroying dyes containing azo links. Thedithionite is normally stored as a solid until required. Commerciallyavailable powdered anhydrous sodium dithionite is stable for longperiods in dry air but undergoes decomposition in the presence of wateror water vapour. The action of water is initially to form the dihydrateof the dithionite with the liberation of heat. The dihydrate isparticularly readily oxidised by atmospheric oxygen, againexothermically. If the heat liberated in these two reactions is notremoved rapidly enough a third exothermic reaction may occur, namely,the spontaneous decomposition of the salt with the disengagement ofsulphur dioxide and the formation of thiosulphate, bisulphite andsulphur. The heat liberated during these reactions may be so great thatthe temperature of the decomposing compositions rises above the ignitionpoint of sulphur, so that the composition finally begins to burn. Sincedithionites are often used in environments of high humidity thistendency to ignite is a serious problem which has in fact given rise toa number of fires in dye-houses and pulpmills.

It is known to reduce the ignitibility of dithionites by admixture ofadditives, a variety of which have been proposed. These previouslysuggested additives which have generally been water soluble materialsvary in their modes of action and degrees of effectiveness. Some merelyfunction as inert diluents to reduce the temperature rise whendecomposition occurs. Other materials function in ways which are notentirely understood to inhibit either the initial or the later stages ofdecomposition. However the amount of stabilizer it is practicable to addis clearly limited, particularly since in many cases the stabilizeritself has a detrimental effect on the process in which the dithioniteis to be used. For example, it has been proposed in W. German OLS1,938,315 to stabilize dithionites by incorporation with a cationic,non-ionic or amphoteric surfactant but surfactants are an embarrassmentin many of the applications in which dithionites are used.

It is also known from B.P. 1,061,299 to stabilize dithionites with oneof a variety of certain specified synthetic polymers having particular Kvalues but such mater'ials are again not satisfactory in all respects.

There is, therefore, a continuing search for fresh stabilizers which areeffective in lower concentrations than 3,840,644 Patented Oct. 8, 1974previous materials and/or are not detrimental in the intendedapplication.

The present invention provides a substantially anhydrous compositioncomprising a major proportion of a metallic dithionite and anon-surfactant nonpolymeric compound containing at least 5 carbon atomswhich comprises at least one group of the structure:

wherein the unsatisfied valencies are filled by oxygen, hydrogen orcarbon atoms. Such compositions show a marked increase in stability todecomposition compared with unstabilized material and may be used inapplications for which surfactant properties would be disadvantageous,by the bleaching of paper pulp.

The stabilizers for use in the compositions of the invention include anumber of preferred categories. A first category consists of compoundswhich may be considered as derivatives of acids RCO H, where H is asubstituted or unsubstituted alkyl or alkenyl group. These stabilizershave the general formula I:

where R is a substituted or unsubstituted alkyl or alkenyl group havingat least 7 carbon atoms; X and Y are the same or different groups beinghydrogen; a group of the formula -(CH CHR'NH) H, where R is hydrogen ormethyl and n is an integer from 1 to 6; a group of the formula -CH(CH'R') NHCOR where mis 0* or 1 and R and R are as hereinbefore defined,R being the same as or different from the other R group or a group ofthe formula CH CH OH or CH CH(OH)CH Alternatively they may be compoundswhich are cyclic derivatives of a compound of the formula as aforesaid,formed therefrom by cyclodehydration, i.e. intramolecular cyclisationwith removal of one molecule of water by condensation, which derivativescomprise a five membered heterocyclic ring containing two atoms otherthan carbon atoms. Stabilizers of the first category in general displaysuperior effectiveness to that of most currently used materials thuspermitting less stabiliser to be used than would be necessary with apreviously known stabiliser.

The group R in aforesaid stabilizers of the first category will normallycontain from 7 to 22, more preferably 8 to 18 carbon atoms and willpreferably be derived from a naturally occurring fatty acid or mixturesof fatty acids such as capric, lauric, myristic, palmitic, stearic,oleic, linoleic or linolenic acid or mixtures thereof such as fattyacids obtained from palm oil, palm kernel oil, coconut oil orhydrogenated tallow. R may bear substituents such as alkyl, cycloalkyl,aryl or hydroxyl, may comprise one or more unsaturated alkenic oralkynic linkages and may comprise either straight or branched carbonchains. R may be derived from synthetic or mineral sources, e.g.naphthenic acids.

The stabilizers of the first category fall into a number of classes.Where X and Y are both hydrogen they are simple acid amides, RCONH Insuch cases it has been found that the most effective stabilisation isachieved when the group R is unsaturated so that the amide has an iodinevalue of at least 45. The presence of an hydroxyl substituent in R alsoappears to enhance stabilisation.

Simple amides are readily obtained by ammonolysis of the correspondingacids or their esters by known procedures.

Stabilizers of the formula I where X is a group of the formula --CH(CHR) NHCOR and Y is H are methylene ethylene, or propylene bis-amides,prepared for example, by reaction of one or more amides, RCONH withformaldehyde, or by reaction of one or more acids, RCO H, withethylenediamine or propylenediamine. Methylene and ethylene bisstearamides and his oleamides are examples of such materials.

Stabilizers of the formula I where X is a group (CH CHRNH) H and Y is Hare obtained by reaction of acid RCO H with ethylene or propylenepolyamines such as ethylene or propylene diamine, diethylene ordipropylene triamine or the like, n being from 1 to 6. Preferably n is 1or 2. Either or both groups X and Y in formula I may also be -'CH CH OHor a CH CH OH CH groups, the compound of formula I being then a mono ordi-ethanolamide or a mono or a diisopropanolamide derivative formed fromreaction of one molecule of the corresponding alkanolamine with acid RCOH.

Stabilizers of the first category may also be five-membered heterocyclicring structures formed by an internal condensation reaction of acompound of formula I as aforesaid. For example a monoethanolamide offormula may be cyclodehydrated by known methods, e.g. straightforwardheating or by heating with an acidic dehydrating agent to yield anoxazoline of the formula useful according to the invention. Also acompound of formula I where X is (CH CHR'NH) H and Y is H may be treatedsimilarly to yield an imidazoline useful according to the invention,e.g. when n is 1 and R is H an imidazoline of the structure A secondcategory of stabilizers for use in the compositions of the invention hasthe formula II II I (I) Y lac-M wherein X and Y are as hereinbeforedefined and R is a substituted or unsubstituted aryl group; or arecyclic derivatives of a compound of the formula II formed therefrom bycyclodehydration, i.e. intramolecular cyclisation with removal of onemolecule of water by condensation, which derivatives comprise afive-membered heterocyclic ring containing two atoms other than carbonatoms.

The group R" may be aryl groups such as phenyl or naphthyl which maybear inert substituents, e.g. alkyl, cycloalkkyl or halo-groups.

Particularly preferred compositions comprising a stabilizer of thesecond category are those comprising as a stabilizer methylene bis(benzamide).

A third preferred category of stabilizers useful according to theinvention are the following derivatives of methylol araylandalkanolamides R"CONHCH OH,

where R is a substituted or unsubstituted alkyl, aryl or alkenyl grouphaving at least 6 carbon atoms:

1. Pyridinium salts of the general formula where X is an anion. Suchmaterials are readily prepared by known means. The compound where R' isC H (stearamidomethyl pyridinium chloride) is well known as an agent inthe water repellent treatment of fabrics.

2. Thiocyanate and phosphate esters of methylol amides R'CONHCH OH.

3. Ethers, preferably water soluble or self-emulsifiable ofmethylolamides R'CONHCH OI-I with hydroxycarboxylic acids andmethyloloxycarboxylic acids, especially hydroxyacetic acid andmethyloloxyacetic acid.

The groups R' in the aforesaid stabilising compounds are substituted orunsubstituted aryl, alkyl or alkenyl groups containing at least 6 carbonatoms. The groups R' which are alkyl or alkenyl will normally contain 7to 22, more preferably 8 to 18, carbon atoms, and will preferably bederived from a naturally occurring fattyl acid or mixtures of fattyacids such as capric, lauric, myristic, palmitie, stearic, oleic,linoleic or linolenic acid or mixtures thereof such as fatty acidsobtained from palm oil, palm kernel oil, coconut oil or hydrogenatedtallow. R may bear substituents such as alkyl, cycloalkyl, aryl orhydroxyl, may comprise one or more unsaturated alkenic or alkyniclinkages and may comprise either straight or branched carbon chains. R'may be derived from synthetic or mineral sources, e.g. naphthenic acids.R'" may also bear substituents rendering the stabilising compound watersoluble. Such substituents may be cationic, anionic or non-ionicsolubilizing substituents such as isothionic acid derived substituents;polyglycol ether substituents, e.g. substituents derived from thereaction products of oleic acid methyolamide and ethylene oxide orpyridinium chloride substituents.

Stabilizers in the third category are generally water soluble materials,many of which are readily soluble in water at room temperature andothers of which are readily soluble at moderately elevated temperaturese.g. C. In some cases the stabilizers are not water soluble but areself-emulsifiable, i.e. capable of forming a uniform dispersion inwater.

Stabilizers in the first and second categories are generallysubstantially insoluble materials. This may be desirable to avoidinterference with any processing operation in which dithionites areused, by the bleaching of paper pulp.

Such materials include ethylene and methylene bisstearamides, tallowacid monoethanolamide and lauric acid diethanolamide. With the exceptionof oleamide simple amides are less preferred.

The quantity of stabilizer employed in the novel compositions willdepend upon its effectiveness and upon the intended lifetime and storageconditions of the composition. The effectiveness of the novelstabilizers, although in general greater than that of previously knownmaterials, may vary within wide limits so that in some cases astabilised composition might contain, say, as little as 0.02% by weightof the composition as stabilizer. In other cases it might be founddesirable to employ up to 5% or in extreme cases, 10% on the same basis;concentrations of from 0.2, more often 0.5%, to 3%, especially 1% to 2%are suitable in the main. The effectiveness of any given stabilizer mayreadily be tested by a standard method involving the addition of waterto a stabilised dithionite under controlled conditions followed by theaddition of a further quantity of the stabilised dithionite andobservation of the ensuing temperature rise. Such a method is describedin the examples given hereinafter.

The novel stabilizers are solid in most cases and may therefore beeasily mixed with a dithionite by known methods, mixing being asintimate as possible. In a few cases the novel stabilizers may be softor sticky low melting solids in which case they may be mixed With'thedithionite dissolved in an organic solvent, e.g., isopropanol, which maysubsequently be evaporated or distilled off. Alternatively thestabilizer may be mixed into the dithionite as a melt. n the other handthe additive may be incorporated into the dithionite during the laststages of dithionite manufacture instead of into the alreadymanufactured dithionite. For example, an alcohol-wet filter cake of adithionite may be mixed with an alcoholic solution of a novel stabilizerbefore drying in a rotary vacuum dryer. Alternatively additive may bedissolved or dispersed in a solution or slurry of a dithionite prior toevaporation to dryness in suitable equipment. If desired a known freeflowing agent such as a sodium phos phate or a sodium salt of acondensed phosphoric acid, soda ash, sodium bicarbonate, sodiumbenzoate, EDTA or, preferably, urea may be added. However, the presenceof a free flowing agent will not normally be necessary.

The inventionis particularly applicable in the stabilisation of sodiumdithionite, but other metal dithionites, such as the potassium orcalcium dithionites may be stabilised similarly. The invention is alsoapplicable in the stabilisation of dithionite products of low strength,for example sodium dithionite products containing from 50% to 90% Of NaS O Stabilizers for use according to the invention are nonpolymericcompounds, which term has its normal significance as connoting acompound containing not more than a few, say 5-10 repeating units. Itwill generally be found that the present stabilizers have molecularweights below 500, or at most, 600. They are also non-surfactantmaterials, by which is meant that they do not significantly reduce thesurface tension of water.

The novel compositions may be used in general in any application inwhich dithionites are at present used, e.g.

.in the "stripping of dyes from textiles, the reduction clearing.of'dyed fibres and in the bleaching of textile materia'l's' and paperpulp. The novel compositions are of particular utility in the bleachingof paper pulp.

The invention is illustrated by the following examples: In the examplesthe effectiveness of various stabilizers was tested by a methodcomprising deposition on the surface of a stabilized dithionite sampleof a quantity of water insufficient to dissolve the sample, furtherdeposition of more stabilized dithionite onto the surface andmeasurement of the ensuing temperature rise. Such a method is novel andconstitutes a further aspect of the invention. The use of one or morethermocouples attached to a recording potentiometer to measure thetemperature rise is preferred. Most preferably a plurality, e.g. 4 to 6,thermocouples are employed, located at different points in thedithionite sample. The particular method adopted for the examples was asfollows:

100 g. of a blend of sodium dithionite and potential stabilizingmaterial in known proportions is placed in a 250 ml. Dewar flask and 7ml. of distilled Water added in one minute by means of a metering pump,the mixture being stirred continuously during this addition. -Sixthermocouples are then introduced into the Dewar to enable temperaturechanges in various parts of the mixture to be observed, all sixthermocouples being connected to a six-point recording potentiometer.After 3 minutes a further 100 g. of the dithionite mixture is added tothe flask and recording of the temperature of the mixture continueduntil it returns to ambient temperature.

A number of the novel stabilizers and materials falling outside thescope of the invention were examined by this method and the results aregiven in Tables I, II and III.

In all cases the temperature of the mixture rose over a period of aboutone to three hours to a value of from around 130 to 140. After thispoint a dramatic variation in behaviour was noted between stabilizedcompositions of the invention (Tables I and II) and compositions admixedwith other materials (Table III). In the former cases, the temperatureof the compositions gradually returned to ambient, but the othercompositions in all cases showed a further rapid rise in temperaturebeyond 130- 140 C. up to 250-350 C. In some cases the sample ignited.This dilference demonstrates the utility of the test 1 method adoptedsince when the other compositions were tested by a method such as usedhitherto, involving the straightforward addition of Water to the sampleand measurement of the ensuing temperature rise without addition offurther dithionite, no temperature rise beyond C. was observed. (SeeTable IV).

Details of the Examples using the test method described to examinestabilized compositions of the inven- 5 tion are tabulated in Table I.

TABLE I Concentration of stabilizer in composition (percent Observedtemperature Ex. Stabilizer \v./w.) effect 1- 12-hydroxy- 2 stearamide.

Composition heated up over 45 minutes to 131 0., remained at thistemperature for ca. 40 mins. then gradually cooled to ambienttemperature. No local ignition occurred.

Temperature rose to 134 C. over 1 hr. 50' mins. and subsequently fell toambient value. No local ignition occurred.

Temperature rose to C. over 1 hr. 30 mins. then gradually fell toambient value. No local ignition occurred.

Temperature rose to 130 C. over 45 mins. then gradually fell to ambientvalue. No local ignition occurred.

Temperature rose to 150 C. over 50 mins. then gradually tell to ambientvalue. No local ignition occurred.

Temperature rose to 130 C. over 45 mins. then gradually tell to ambientvalue. No local ignition occurred.

Temperature rose to 146 C. over 55 mins. then fell to ambient value. N 0local ignition occurred.

Temperature rose to 140 C. over 1 hr. 45 mins. then gradually fell toambient value. No local ignition occurred.

Temperature rose to 132 C. over 50 mins. then gradually fell to ambientvalue. No local ignition occurred.

Temperature rose to 133 C. over 1 hr. then fell to ambient value. Nolocal ignition occurred.

Temperature rose to C. over 1 hr. 10 mins. and then gradually fell toambient value. No local ignition occurred.

2.- Stearamide 10 3. Palm kernel fatty 2 acids monoethanolamide.

4 Ethylene bis 2 stearamide.

6 Methylene bis 2 stearamide.

8. Oleamide 2 9- Lanolin fatty 2 acids amide.

10- Lauric acid 1 diethanolamides 11.--- Laurie acid di- 1 ethanolamidewith excess free ethanolamine (manufactured by Kritchevsky reaction).

12. Tallow fatty acids 1 monoethanolamide.

Temperature rose to 135 C. over 1 hr. then gradually tell to ambientvalue. N o ignition occurred.

Temperature rose to 145 C. over 1 hr. 20 mins. then fell to ambientvalue. No local ignition occurred.

TABLE IContlnued Concentration of stabilizer in composition I (percentObserved temperature Ex. Stabilizer w./w.) effect 15. Oxazoline as sold2 Temperature rose to 149 under the reg- C. over 1% hrs. then isteredtrade gradually tell to ambient name Crodvalue. No ignition. azoline S(PP 618).

16. Methylene bis 2 Temperature to 134 (stearamide). C. over 55 minutesthen fell to ambient value.

17. Sodium oleamido- 2 Temperature rose to 131 methane C. over 70 mins.then sulphonate. gradually tell to ambient. No local ignition occurred.

18 .do 2 Temperature rose to 144 C. over 2 hrs. 15 mins. then graduallyiellto ambient. No local ignition occurred.

19. A stearamido- 2 Temperature rose to 137 methylpyrid- Cover 1 hr. 5nuns. idinium chlothen gradually fell to ride sold under to ambientvalue. No the trade name local ignition occurred. Velan PF(approximately 60% active) 20. As Example 19...- 3 Temperature rose to120 C. over 33 mins. then gradually fell to ambient value. No localignition occurred.

21 "do 1 Temperature rose to 131 C. over about 30 mins. then graduallytell to ambient value. No local ignition occurred.

22 do 2 Temperature rose to 129 C. over 43 mins. then gradually fell toambient value. No local ignition occurred.

These Examples employed low strength (60-70%) Na2s20-l and the testmethod was modified by use of 6 mls. water instead of the usual 7 mls.

In a further series of tests a variety of stabilizers of the inventionwere examined by the method used in the Examples of Table I. In allcases the temperature of the mixture of sodium dithionite and stabilizerrose to a maximum then gradually returned to ambient value. Thestabilizers tested in the given concentrations are shown against themaximum test temperature attained in Table II.

TABLE II Maximum Concentration temperaof stabilizer ture in mixtureattained Ex. Stabilizer (percent w./w.) C.)

23- Methylene bis (stearamide) 2 126 24- Maleanilic acid 1 127 25-PhNHCOCH=CHCOO 1 125 26 Sodium phthalanilate 1 125 27.--- -C6H4 (COONa)(CONHPh 1 125 28- Disodium stearamidomethane 1 126 phosphonate. 29.Aeetanilide 2 126 30--" PhNHCOCH; 2 138 3l Phthalimido-acetic acid 1 12632- N-methylenecarboxyphthalimide. 1 124 33. Steargmidomethanephosphonic 1 131 aci 34---. CH3(CH2)1CONHCHPO(OH); 1 130 35-N-phenylurea 2 132 36 PhCONHNHL- 1 125 37 2-imidazolidone 1 126 38----CH-NH 1 127 0:0 0 Hr-NH 39- N-2hydr0xyethyl phthalimide 1 124 40 N-(B-hydroxyethyl)phthalimide..- 1 125 Samples of sodium dithioniteadmixed with materials other than stabilizers of the invention were thenexamined for stability by the novel tcst method. Results are given inTable III.

TABLE III Concentration of admixed material Observed temperature Ex.Admixed material (percent w./w.) efi'cct 4l Polyvinyl-pyrroli- I 2Temperature rose to 142 C. over 55 mins.

done, k-value=30.

then very quickly to 362 C.

42. PVP-co-polymer as 2 Temperature rose to sold under name 148 C. over65 AntaroxV 904." mins., then very 43. Alginic acid grade 5 Temperaturerose to HS/LD (Alginic 132 C. over 60 min., Industries Ltd.). then, morerapidly to 426 C., with ignition.

44 Methyl cellulose 5 Temperature rose to powder as sold 150 C. over 60mins., then, more under trade name rapidly, to 458 C.

Celacol M10000 DS.

with ignition. 45 High molecular 5 Temperature rose to weight ethylene135 C. over 60 mins., then, more rapidly, to 433 0., with ignition.

oxide polymer as sold under trade name Polyox Coagulan by Union CarbideLtd Temperature rose to 133 C. over 70 mins., then, more 46....Polytmethyl vinyl 2 ether) maleic anhydride resin as sold under traderapidly, to 435 C. name Gantrez with ignition. AN 119.

47 Water soluble 5 Temperature rose polyaerylio acid to 169 C. over 60k-value=116, as mins. then, very sold under trade rapidly to 350 C. nameVersicol EN 23 by Allied Colloids Ltd.

Temperature rose to 143 C. over 60 mins., then, to 391 C., withignition.

48- Polyvinyl alcohol powder as sold under trade name Collasyn TA byAllied Colloids Ltd.

TABLE IV Concentration of stabilizer in composition Observed temperature(percent) eflect Ex. Stabilizer 49 As in Example 42. Temperature rose to125 C. and then gradually tell. No ignition.

Temperature rose to 129 C. then fell. N0 ignition.

Temperature rose to 120 C. then fell. No ignition.

50- As in Example 45 51 As in Example 46.

As a further comparison the test method of Table I was carried out on ag. sample of sodium dithioniite as used previously to which nostabilizer had been added. The temperature of the mixture rose to 131 C.over 50 minutes followed by a sudden and very rapid rise to 320 C.

We claim:

1. A substantially anhydrous composition comprising a major proportionof a metallic dithionite and an effective amount of' a stabilizertherefore which is a nonsurfactant compound containing at least 5 carbonatoms and at least one group of the structure:

wherein the unsatisfied valencies are filled by oxygen, hydrogen orcarbon atoms, said compound being monomeric or containing not more than10 repeating units and having a molecular weight of up to 600.

2. A composition as claimed in claim 28 wherein the stabilizer has thegeneral formula 1 where R is a substituted or unsubstituted alkyl oralkenyl group having at least 7 carbon atoms; X and Y are the same ordifferent groups being hydrogen; a group of the formula -(CH CHRNH) H,where R is hydrogen or methyl and n is an integer from 1 to 6; a groupof the formula CH (CHR') NHCOR where m is 0 or 1 and R and R' are ashereinbefore defined, R being the same as or different from the other Rgroup or a group of the formula CH CH OH or CH CH(OH)CH 3. A compositionas claimed in claim 2 wherein the group R contains from 7 to 22 carbonatoms.

4. A composition as claimed in claim 2 wherein the stabilizer is tallowacid monoethanolamide.

5. A composition as claimed in claim 2 wherein the stabilizer is lauricacid diethanolamide.

6. A composition as claimed in claim 2 wherein X in formula I is a groupCH (CHR)NHCOR and Y is hydrogen.

7. A composition as claimed in claim 6 wherein the stabilizer isselected from the group consisting of methylene bis stearamide, ethylenebis stearamide, methlyene bis oleamide and ethylene bis oleamide.

8. A composition as claimed in claim 2 wherein X in formula I is a group(CH CHRNH) H, wherein n is from 1 to 6, and Y is hydrogen.

9. A composition as claimed in claim 8 wherein n is 1 or 2.

10. A composition as claimed in claim 2 wherein at least one group X andY in formula I is CH CH OH or CH CH(OH)CH 11. A composition as claimedin claim 28 wherein the stabilizer has the formula II II Y wherein X andY are as hereinbefore defined and R" is a substituted or unsubstitutedaryl group.

12. A composition as claimed in claim 11 wherein the stabilizer ismethylene bis(benzamide).

13. A composition as claimed in claim 28 wherein the stabilizer is aderivative of a compound of formula R"CONHCH OH, wherein R is asubstituted or unsubstituted alkyl, aryl or alkenyl group having atleast 6 carbon atoms, said compound being a pyridinium salt, athiocyanate or phosphate ester or an hydroxycarboxylic acid ether ofsaid compound of formula 14. A composition as claimed in claim 13wherein the stabilizer is a water soluble or self-emulsifiable ether ofa hydroxycarboxylic acid.

15. A composition as claimed in claim 13 wherein the stabilizer is anether of hydroxyacetic or methyloloxyacetic acid.

16. A composition as claimed in claim 13 wherein the group R in thestabilizer is an alkyl or alkenyl group containing from 7 to 22 carbonatoms.

17. A composition as claimed in claim 1 wherein the stabilizer ispresent in an amount of from 0.02 to 10% by weight on the composition.

18. A composition as claimed in claim 28 wherein the stabilizer ispresent in an amount of from 0.5 to 3% by weight on the composition.

19. A composition as claimed in claim 2 wherein the stabilizer ispresent in an amount of from 0.02 to 10% by weight on the composition.

20. A composition as claimed in claim 19 wherein the stabilizer ispresent in an amount of from 0.5 to 3% by weight on the composition.

21. A composition as claimed in claim 11 wherein the stabilizer ispresent in an amount of from 0.02 to 10% by weight on the composition.

22. A composition as claimed in claim 13 wherein the stabilizer ispresent in an amount of from 0.02 to 10% by weight on the composition.

23. A composition as claimed in claim 22 wherein the stabilizer ispresent in an amount of from 0.5 to 3% by weight on the composition.

24. A composition as claimed in claim 1 which further comprises a freeflowing agent.

25. A composition as claimed in claim 1 wherein the dithionite is ofsodium, potassium or calcium.

26. A process for preparing a composition as claimed in claim 1 whichcomprises admixture of the dithionite with a solution of the stabilizerin an organic solvent and subsequent evaporation of the solvent.

27. A process for preparing a composition as claimed in claim 28 whichcomprises mixing the dithionite with a melt of the stabilizer.

28. A composition as claimed in claim 1 wherein said stabilizer has amolecular weight below 500.

References Cited FOREIGN PATENTS

1. A SUBSTANTIALLY ANHYDROUS COMPOSITION COMPRISING A MAJOR PROPORTON OFA METALLIC DITHIONITE AND AN EFFECTIVE AMOUNT OF A STABILIZER THEREFOREWHICH IS A NOMSURFACTANT COMPOUND CONTAINING AT LEAST 5 CARBON ATOMS ANDAT LEAST ONE GROUP OF THE STRUCTURE: